Oversaturated CCD?

[laser_jock99]

Is it possible to oversaturate / overexpose a DSLR image. I have wierd things going with some subs that are longer than 10 minutes on very bright subjects.

Normally anything overexposed I would expect to burned out white- but in these two images (and others) I got odd results which will not process out.

[red dwalf]

I'm not 100% certain but I usually go by my histogram, I look for it to be somewhere around a quarter of the way along the graph from the left hand side, but it's very easy to overexpose brighter objects, 5 minutes Max I would say would be best.

[AndyUK]

Yes, you certainly can oversaturate a DSLR chip.... but I'd agree that the best way to assess it is by looking at the Histogram on the back of the camera - I too used to aim for the 3 peaks to be around 1/4 to 1/3 across from the left hand side... 

I also used to use an EOS Clip filter (my LP's not great ), but "max" exposure really depends on the DSO.  The trapezium on M42 is obviously REALLY bright, whereas the outer areas you can get more detail by increasing the exposure.  I just checked an image I took a few years back (@ f5.3) for which I used 2 sets of exposures - 300s for the main image and 30s exposures for the trapezium - and merged the two together with a gaussian blurred layer mask.

You can certainly take 10min exposures on some DSO's with a DSLR (for example, I recall I did for the Rosette), but you then need to be wary of the chip heating up, especially in the summer, and then of course darks come into play....

[laser_jock99]

Yes, you certainly can oversaturate a DSLR chip.... but I'd agree that the best way to assess it is by looking at the Histogram on the back of the camera - I too used to aim for the 3 peaks to be around 1/4 to 1/3 across from the left hand side... 

Histogram is mainly in the third quarter- so possibly just overexposed.

[DaveS]

If you're shooting raw then the display should flash "blinkies" where you're overexposing. On the histogram you'll see a spike at the far right.

[laser_jock99]

If you're shooting raw then the display should flash "blinkies" where you're overexposing. On the histogram you'll see a spike at the far right.

I don't think this is overexposure- there's now way of even recovering the RAW files. Something else is going on- possibly to do with the dynamic range settings I was dabbling with on the camera?

This camera can go for 1200s @ ISO3200 and still be recovered in RAW editor.

1200s @ ISO3200 RAW image

The same file but 'recovered' in Photoshop RAW editor.

[AndyUK]

Can you supply the full details for the image (# of exposures, ISO, calibration frames(?), scope and also how you processed it (both registering/stacking and processing applications)...?

I see from another thread that you have a Fuji DSLR(?) - I'm unfamiliar with that one, but when I had a Canon 40D, I just used it on full manual/bulb mode, with a remote timer, and let it take as many frames as possible for as long as possible.  I really should have asked what ISO you were using, as obviously that has an impact on exposure too(!) - I always used to default to ISO800 (except for framing) and aimed to try and take at least 3 hours worth of exposures (then double that before I made the move to CCD )

(Although that single "recovered" 20 min RAW at ISO3200 has come out pretty well, the raw image itself does look pretty over-exposed...)

[laser_jock99]

Having had a further play last night it might actually be a computer software issue and not a camera problem after all.

Normally I use 32 bit Adobe Photoshop Elements 12 to open and do the initial RAW editing (exposure/colour balance/noise etc.) prior to stacking and post processing in TIFF format.

However, I have discovered that my 64bit version of Corel Paintshop Pro X6 can open the RAW files and do the initial editing better. It may be that these particular raw files have addition bit depth or dynamic range information that Elements can't cope with?

[AndyUK]

Normally I use 32 bit Adobe Photoshop Elements 12 to open and do the initial RAW editing (exposure/colour balance/noise etc.) prior to stacking and post processing in TIFF format.

Ah - The "usual" process is to calibrate/register/stack the RAW frames before doing any editing...

If you're using DSS for calibration/registration/stacking(?), you'd then save the output as a .tiff file with settings embedded and then take the stack into your post-processing software.

(However, as I noted, I'm unfamiliar with FUJI RAW frames - I assume DSS can read them?  If not they'd first need to be converted to .tiff (with no additional processing though), and then brought into DSS).

If you have a dropbox(?) and can upload all your untouched RAW frames (and any calibration frames - I know they'll be fairly big and it will take a while!) and can post a link to the folder, I'd happily have a look at it if you wish?

[ollypenrice]

The first image you posted looks posterized. This arises from the colour depth not being sufficient to produce a smooth series of gradations so I suppose this might arise from overly long exposures. I'm not up on DSLR imaging, really.

Olly

[laser_jock99]

I've come to the conclusion the subs are just beyond rescue. Ten minutes exposure is too long on these bright objects.

[StuartJPP]

I find that it is quite easy to overexpose/clip bright objects such as the cores of M42 and  M31 as well as stars. I have noticed that with Canon cameras if you overexpose then try to recover often a magenta tint is present instead of pure white. The best option is to ensure that you are not clipping the data in the first place.

The 1200s image at ISO3200 posted above isn't necessarily clipped. The region of sky captured looks pretty flat and uniform and is probably much, much dimmer in comparison to M42 so it is probably overexposed but not clipped. In other words the dynamic range isn't massive like it is in the M42 image.

[MattJenko]

I have done 10 minutes with a Canon 450d on M42 without too much trouble. The core was whiter than white and the red histogram was well over halfway to the right, but the image was processable, and was one of my best images. I was shooting using a low ISO of 400 though, were you doing 10 mins at 3200?